1. Field of the Invention
The present invention relates to a document reading apparatus including an optical system configured to project an image of a document to an image pickup device, and an image formation apparatus including a document scanner configured to drive one of the optical system and the document in relation to the other in a sub-scanning direction, and an image data processor configured to compensate for distortion of read image data.
2. Description of the Related Art
As for apparatuses such as a copier, a facsimile apparatus, and a scanner that read an image using an optical line sensor, there are two types. Namely, in a flat bed method, a document is placed on a reading glass (contact glass), and an image of the document is read by either driving a luminous source and a mirror for projecting the image to a fixed line sensor, or by moving the light source and the line sensor at a fixed speed; and in a sheet feed method, the image is read by moving the document while the light source and the line sensor are fixed.
If a reading operation (henceforth called scanning) of a document is performed with dust adhering to the reading glass, the image of dust is read as a point of the same magnitude as the dust in the case of the flat bed method. Consequently, a point-like foreign substance appears at the position of the dust on the read document image. In the case of the sheet feed method, the image of the dust on the reading window on which the document passes is read as a noise line having a width equal to the size of the dust. Consequently, in the read document image, a foreign substance image (noise line) appears dividing the document image. An example of the foreign substance image (noise line) is shown in FIG. 16.
If the size of the dust is very small such as dust drifting in the air, the point-like foreign substance image will hardly degrade the quality of the image. However, a straight line-like foreign substance image, i.e., a noise line, degrades the appearance of the image regardless of the size of the dust, since the noise line is present for the full length of the image. Even if the flat bed method is employed, wherein the luminous source and the mirror move in the sub-scanning direction for projecting a document image to the fixed line sensor, if the mirror that reflects the light of the document to the image pickup device is dirty, a noise line will appear.
If the cause of the foreign substance image is dust, the problem can be solved by cleaning the reading window when a noise line is found on the read document image. However, in the case of facsimile transmission, and the like, transmission is often carried out to a partner without the user checking an image, and without noticing a noise line. Further, when a great number of documents are copied, and when a great number of copies of a document are carried out using a copier, cleaning after noticing the noise line can be time and effort consuming. In order to cope with this problem, image readers that are capable of reducing a noise line to an inconspicuous level are available, wherein image data processing is performed. Conventional technologies are described.
[Patent Reference 1]
JPA 10-233875 proposes a noise line detection method that determines presence of a black stripe, and displays that there is an unusual image. According to the method, a scanner reading image is rotated 90 degrees (i.e., the main scanning direction and the sub-scanning direction are exchanged), and the rotated image is stored in a memory. Then, continuation of black pixels in the main scanning direction is detected, and it is determined whether the continuation exceeds a predetermined value. If the determination is affirmative, it is determined that there is a black stripe, and a notice to the effect is displayed.
[Patent Reference 2]
JPA 2001-144901 discloses a noise line detection method, wherein a conveyance belt is read without conveying a document, whether there is a black stripe in the read image is determined, and an alarm signaling that the reading glass is dirty is displayed if the determination is affirmative.
[Patent Reference 3]
JPA 2003-169214 discloses an image formation apparatus, wherein multi-value image data of a document are converted into binary data, the binary data are compared with a matching pattern beforehand stored to a ROM such that a black stripe is detected by pattern matching, and the black stripe is removed.
[Patent Reference 4]
JPA 2003-198838 presents an image processing method of detecting a belt-like image as a noise area, the belt-like image beginning with the starting edge of document reading and ending with the ending edge of the document, and rectifying the image data of the noise area based on pixel values of adjacent pixels.
As described above, the conventional technologies detect a noise line by determining whether unevenness in gradation data in the main scanning direction matches a noise line, based on the gradation data of a read image or binary data thereof. However, so long that the presence of a noise line is determined by the unevenness in the gradation data, there is always a risk of mistaking a line originally written on the document for a noise line. If such a mistake occurs, a part of the document is not reproduced, the part being erroneously determined to be noise, and readability of the reproduced image is degraded.
However, if an attempt is made such that a document image and a noise line are faithfully distinguished, processing steps and the configuration of an image reading apparatus become complicated. For example, although it is possible to store image data of a page in a frame of memory, to read the data from the frame of memory, to determine the presence of a noise line, and to compensate for image data of the pixels of the noise line without spoiling original data of adjacent pixels, a great memory capacity for storing the one page of image data is needed. Especially, when the image is read in multi-gradation (for example, 8 bits), a great amount of memory is needed, and further, the processing time becomes remarkably long. From the background as described above, a method of detecting a noise line on an image expressed by multi-value image data in a short time from the start of document reading using a small buffer memory is required.